Technically, you can switch to 12/12 anytime...it all depends on height and yield restrictions/capabilities...usually, a foot or more ( height for plant) is the common cinsensus for flowering. As of today, I'm 6 weeks into vegging, and if i didn't LST, mine would be well over a foot tall...but i m growing one, and want to yield a good amount. I have approx 20 tips, with plenty more on the way up!